Even in the earliest moments of his 2012 re-election campaign, Florida Sen. Bill Nelson can see the windup. Twice before Republicans tried to discredit him as a liberal. Twice they failed.

"Facts," Nelson says, "are stubborn things."

But the ground has shifted and Nelson is a bigger target than ever — Florida's last remaining statewide-elected Democrat, a reliable vote for President Barack Obama and a speed bump in Republican plans to finish the job they started in the midterm elections to seize the Senate.

"Marco Rubio's election showed that Republicans certainly can, if they field a good candidate, pick up that seat," said Sen. John Cornyn, the Texan overseeing the GOP's Senate election effort.

One major Republican has entered the race, Florida Senate President Mike Haridopolos. He and other potential rivals have been nipping at Nelson for months.

"He's a very good politician," Haridopolos said. "He sounds like John Wayne. He dresses like a conservative. He votes liberal."

U.S. Rep. Connie Mack of Fort Myers went a step further and called Nelson "ultra-liberal" in a fundraising appeal to supporters.

"Let's tell the professional politicians like Florida's Senior Senator, incumbent Democrat Bill Nelson, that we've seen their liberal record, heard their liberal speeches, and seen their liberal results and we aren't fooled — and we won't forget."

In 2000, Nelson faced the same charges from Bill McCollum, and in 2006 it was Katherine Harris who uttered the L word every other breath — four times in one speech lasting less than four minutes. McCollum came up short and Harris lost by a landslide.

McCollum was perhaps too conservative for the time, and Harris ran a disastrous campaign. A more formidable opponent may be able to wield the liberal stick with more force.

"In some ways, Nelson has been blessed by a lot of good luck. He may have (in 2012) the first real race he's had," said Jennifer Duffy, who studies Senate races for the nonpartisan Cook Political Report.

Nelson, 68, sits back in his chair and smiles as he repeats the words, "Facts are stubborn things."

• • •

The facts, at first blush, underscore Nelson's vulnerability.

He has voted for every major initiative of Obama's tenure: the $787 billion stimulus, Wall Street reform, a repeal of the military's "don't ask, don't tell" policy and the health care overhaul.

Last year, Nelson voted with Democrats 89 percent of the time, down slightly from 2009, according to an analysis by CQ Politics. He gets very high ratings from labor unions and very low ones from antitax and gun rights groups.

Yet a deeper voting history and other benchmarks, such as his work on committees and public image, reveal a hyper-cautious politician who gravitates to the center.

In 2009, Nelson ranked as the 39th-most liberal member of the Senate and the 60th-most conservative, according to a National Journal analysis. That puts him in the centrist camp. Longer-range studies show the same.

"I'm a political moderate," Nelson said. "In the mainstream of American politics and the mainstream of Florida politics."

Nelson broke with his party early in his first term to repeal the estate tax but opposed the Bush-era tax cuts (in November, though, he voted to extend them as part of a compromise Obama forged with Republicans). He voted in 2008 for a major expansion of the government's surveillance powers but against the bank bailout.

It's a formula that has irritated fellow Democrats at times — most recently when he dragged his feet on whether to support the "public option" for health care — but one Nelson has practiced over four decades to become one of Florida's most successful politicians.

Elected to the state Legislature in 1972 from a district in Melbourne that voted overwhelming for Richard Nixon, Nelson has lost only one race, the 1990 Democratic primary for governor.

"He's fairly undefined. The upside is he can define himself. The downside is Republicans can draw that picture, too," said Duffy of the Cook Political Report.

"He's not excessively out of the mainstream philosophically," acknowledged Tom Slade, the former head of the Republican Party of Florida. "He's tiptoed just as close to the middle of the line as he can get."

"But he is far from what Florida needs in the U.S. Senate," Slade added. "He's a brick or two short of a full load. He's just lukewarm."

• • •

Longer than he has endured the liberal salvos, Nelson has had to contend with a perception he is a bit of a lightweight, a lawmaker who takes the easiest path of resistance, still reveling in the fame of his voyage into space in 1986.

His weakness may also be his strength. By avoiding sharp definition, Nelson is able to appeal to more voters in a closely divided state with a lot of swing voters.

Nelson can stand with hard-core Democrats in South Florida, where he was born, but he can also slip on cowboy boots and visit North Florida to talk about his Panhandle ancestors, a deep baritone imparting Southern credibility.

He chafes at the empty suit perception and he points to his role in big issues confronting Florida. For years Nelson has played a central role in preventing the encroachment of oil rigs near Florida and openly criticized the Obama administration for its handling of the BP oil spill.

He has long been involved in NASA issues and fought Obama's plan to phase out trips to the moon. Nelson crafted a compromise last year that gave $1.6 billion to commercial space companies but required NASA to build a new moon rocket, saving some jobs.

"He's effective on everything," said Sen. Jay Rockefeller, D-W.Va., who serves on several committees with Nelson and deems him a "classic moderate."

Last week provided a fresh example of how Nelson operates. Hours before a vote on a measure to repeal the health care overhaul, a string of well-known Democrats took to the Senate floor in vigorous defense of the law.

When it came time for Nelson, he split the middle: some parts are good, others can be fixed, he said. Then he offered a resolution urging the U.S. Supreme Court to weigh in as soon as possible.

The next day Nelson went on a Washington news show and said that it was a "possibility" the court could rule part of the law unconstitutional but not a "probability."

You could practically hear his Democratic colleagues groaning across Capitol Hill.

• • •

In his office last week, Nelson grew impatient with questions about his voting record.

"Why aren't you in here talking to me about Egypt right now?" he asked. The day before Nelson became one of the first Democrats to call for President Hosni Mubarak to step down, and he was eager to display the assertiveness.

But the topic was the campaign and the liberal attacks already coming his way nearly 22 months before the election. "They tried that in 2000 and they tried it again in 2006," Nelson said.

"The truth is a pretty good defense."

He has quietly cranked up his campaign and starts the cycle with a healthy $3.1 million in the bank. Nelson has something else on his side: A divided Congress and Obama's own need to regain independent voters will mean less partisan issues in the next two years, more compromise.

He is confident Obama will win. As for himself: "I assume nothing. I run like there's no tomorrow."

Alex Leary can be reached at leary@sptimes.com. Follow him on Twitter @learyspt.